1. Field of the Invention
The invention relates in general to a backlight module, and more particularly to a direct type backlight module using combined types of lamps.
2. Description of the Related Art
Due to features such as slim size, low weight, low power consumption and zero or near zero radiation pollution, liquid crystal displays (LCD) have been widely used recently. An LCD includes a display unit, a backlight module, and a housing having a bracket. The display unit includes a display panel, a circuit board capable of processing data signals and a plurality of tape carrier packages (TCPs) connected therebetween.
The backlight module positioned under the display unit provides a uniform light source to the display unit of the LCD. The light source can be an edge type backlight module or a direct type backlight module. Generally speaking, products for small applications, such as notebooks or mobile phones, normally use edge type light sources, e.g., CCFLs(cold cathode fluorescent lamps) or LEDs(light emitting diodes) together with an optical plate such as a light guide plate, a diffuser film or a light enhancing film disposed on the light source to improve the light uniformity and illumination efficiency. In a direct type backlight module, the light source is disposed under the LCD to produce a better source of light to the LCD. TV has a higher standard of requirements in terms of brightness and wide-view angle. Therefore, the direct type light sources will become the mainstream light sources for large-scaled monitors and TVs. Compared to an edge type light source, a direct type light source employs a larger number of lamps. However, the light of the direct type light source is projected right from underneath the LCD, and hence, the planar uniformity of the light can be achieved by a diffuser plate or a diffuser film without any light guide plate.
FIG. 1A shows an exploded, perspective view of a conventional direct type backlight module. Conventional direct type backlight module 10 includes a plastic housing 5, a bezel 11, a reflective sheet 12, a plurality of lamps 13, a diffuser plate 14, and a plurality of diffuser sheets 15. The lamp 13 can be a cold cathode fluorescent lamp (CCFL), for example. The bezel 11 has an accommodation recess formed therein. The reflective sheet 12 is disposed on the bottom of the accommodation recess and on the wall of the two lateral sides of the accommodation recess. The diffuser plate 14 and the diffuser film 15 are used to convert linear light sources produced from the CCFLs 13, arranged in parallel in the accommodation recess, into a planar light source. The reflective sheet 12 disposed inside the bezel 11, a metal film or an aluminum plate, for instance, reflects the stray light to the image displaying surface to enhance the overall light efficiency. The backlight module 10 is received in the plastic housing 5 first and then is assembled with the display unit (not shown in the diagram) to form an LCD.
FIG. 1B is a schematic top plan view, which shows the arrangement of lapms in the conventional backlight module. Generally speaking, the lamps 13 of a conventional backlight module are horizontally disposed on the reflective sheet 12 and are arranged in parallel to each other at a fixed interval. In the conventional backlight module, electrodes (not shown) at theends of the lamps 13 are connected to an inverter 16, which converts the DC voltage into an AC voltage to drive the lamps 13. When an external voltage is applied to the electrodes positioned on the two ends of each of the lamps 13, electrons released from the electrodes excite an inert gas, e.g. mercury, inside the lamp to emit UV radiation. When the UV radiation projected on the phosphor pasted on the inner side of the lamp, visible light is generated. Along with the fast growth in the liquid crystal electronic appliances, the demand for large-scaled LCDs also increases, requiring lamps of greater length.
However, horizontally disposed long lamps might be easily bent or ruptured due to gravity. Moreover, the accompanied problems of a greater temperature difference and more uneven distribution of mercury in long lamps all cause the luminance uniformity of the lamps to deteriorate. Furthermore, a bent and fragile lamp would result in unevenly distributed backlight, and severely jeopardizing the display quality of the LCD. Besides, because a long lamp requires a larger starting voltage and a larger operating voltage, not only the transformer for the inverter needs to be redesigned, but also a too large voltage difference between the two ends of the lamp would result in uneven luminance of the lamp. Therefore, how to develop a backlight module which has excellent display quality and is easy to be driven for a large-scaled monitor has become an imminent challenge nowadays.